Ion implantation is a standard technique for introducing conductivity-altering impurities into substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the bulk of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity.
Solar cells provide pollution-free, equal-access energy using a free natural resource. Due to environmental concerns and rising energy costs, solar cells, which may be composed of silicon substrates, are becoming more globally important. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Doping may improve efficiency of solar cells. FIG. 1 is a cross-sectional view of a selective emitter solar cell 210. It may increase efficiency (e.g. the percentage of power converted and collected when a solar cell is connected to an electrical circuit) of a solar cell 210 to dope the emitter 200 and provide additional dopant to the regions 201 under the contacts 202. More heavily doping the regions 201 improves conductivity and having less doping between the contacts 202 improves charge collection. The contacts 202 may only be spaced approximately 2-3 mm apart. The regions 201 may only be approximately 100-300 μm across. FIG. 2 is a cross-sectional view of an interdigitated back contact (IBC) solar cell 220. In the IBC solar cell, the junction is on the back of the solar cell 220. The doping pattern is alternating p-type and n-type dopant regions in this particular embodiment. The p+ emitter 203 and the n+ back surface field 204 may be doped. This doping may enable the junction in the IBC solar cell to function or have increased efficiency.
In the past, solar cells have been doped using a dopant-containing glass or a paste that is heated to diffuse dopants into the solar cell. This does not allow precise doping of the various regions of the solar cell and, if voids, air bubbles, or contaminants are present, non-uniform doping may occur. Solar cells could benefit from ion implantation because ion implantation allows precise doping of the solar cell. Ion implantation of solar cells, however, may require a certain pattern of dopants or that only certain regions of the solar cell substrate are implanted with ions. Previously, implantation of only certain regions of a substrate has been accomplished using photoresist and ion implantation. Use of photoresist, however, would add an extra cost to solar cell production because extra process steps are involved. Other hard masks on the solar cell surface likewise are expensive and require extra steps.
Implanting a substrate through a mask, such as a shadow or proximity mask, has drawbacks. First, the throughput of the ion implanter is reduced if the substrate is implanted through a mask because some of the ion beam is blocked. Second, masks are difficult to cost-effectively manufacture, especially with small aperture sizes. Third, the mask itself may be fragile due to the size of the apertures. If supports or solid mask portions between the apertures weaken, then the aperture may not align to the desired regions of the substrate. Poor implant region placement, poor dimensional tolerance, thermal expansion, or damage to the mask may result during implantation. Fourth, the use of a mask produces regions of two doses: a first region having the implant dose and a second region having zero dose. It may be desirable to have a more variable level of dosing in alternating striped patterns for some applications. However, to do a blanket implant across an entire face of a substrate and then a selective implant using a mask requires repositioning of either the mask or substrate. This reduces throughput of the implanter, adds complexity to the implanter, and reduces the fidelity of the implant pattern. Accordingly, there is a need in the art for an improved method of implanting through a mask and, more particularly, a method of moving a mask to perform a patterned implant of a substrate.